The authors present a search for the production of excited or exotic muons ({mu}*) via the reaction {bar p} + p {yields} {mu}* + {mu} {yields} {mu}{gamma}+{mu} using 371 pb{sup -1} of data collected with the Run II CDF detector. In this signature-based search, we look for a resonance in the {mu}{gamma} mass spectrum. The data are compared to standard model and detector background expectations, and with predictions of excited muon production. We use these comparisons to set limits on the {mu}* mass and compositeness scale {Lambda} in contact interaction and gauge-mediated models

/Illinois U., Urbana

Gerberich, Heather

Hays, Christopher

Kotwal, Ashutosh

U., /Duke

U., /Oxford

UNT Digital Library

Search for Excited and Exotic Muons at CDFHeather Gerberich   , Christopher Hays† and Ashutosh Kotwal   University of Illinois, Urbana, Illinois 61801†University of Oxford, Oxford OX1 3RH, United KingdomDuke University, Durham, North Carolina 27708Abstract. We present a search for the production of excited or exotic muons (µ  ) via the reaction p¯  p  µ   µ  µγ  µusing 371 pb  1 of data collected with the Run II CDF detector. In this signature-based search, we look for a resonance inthe µγ mass spectrum. The data are compared to standard model and detector background expectations, and with predictionsof excited muon production. We use these comparisons to set limits on the µ  mass and compositeness scale Λ in contactinteraction and gauge-mediated models.Keywords: excited, exotic, muonPACS: 12.60.Rc, 13.85.Qk, 14.60.HiINTRODUCTIONIn the standard model (SM), quarks and leptons are considered fundamental particles. An indication that quarks andleptons are composite particles would be the observation of their excited states [1]. Additionally, when the standardmodel is embedded in larger symmetry groups, exotic fermions are predicted [2]. We search for singly producedexcited and exotic muons where the µ  decays in the µγ channel, resulting in a µµγ final state signature. The µµγsignal is fully-reconstructible with low background expectation.EXCITED AND EXOTIC MUON MODELSWe consider two models for excited and exotic muon production: a contact interaction (CI) model and a gaugemediated (GM) model. In the CI model, excited muon production is described by a four-fermion Lagrangean of quarksto excited and SM muons [1]. The CI cross sections depend on the µmass Mµ  and compositeness scale Λ. The CIprocess is modeled by PYTHIA [3]. The production of µin the GM model is described by its coupling to gauge bosons[4]. The GM cross sections depend on Mµ  and f 	 Λ, where f is a phenomenological coupling constant. The programsLANHEP [5] and COMPHEP [6] are used to calculate leading order GM cross sections and generate GM events. Forboth models, the µdecays are prescribed by the GM Lagrangean [7].DATASET AND SIGNAL SELECTIONWe use 371 pb  1 of data collected with the high pT muon trigger at CDF from February 2002 through September2004. We search for events consisting of two muons and a photon. The isolated muons must have pT  20 GeV/c andbe located in the central portion of the detector ( η  1), with at least one detected in the muon chamber. Muons areidentified by their minimum-ionizing particle properties. The isolated photon must have ET  25 GeV, can be locatedin the central or forward region, and is identified by its electromagnetic shower properties. In addition, we veto eventswith 81  Mµµ  101 GeV/c2, to remove events produced by initial-state radiation (ISR) p  p¯  Z  γ .TOTAL SIGNAL ACCEPTANCEThe total signal acceptance is measured using the GEANT[8]-based CDF detector simulation. The CI total signalacceptance increases from 13% at Mµ100 GeV/c2 to an asymptotic value of 21% for Mµ400 GeV/c2. For the)2 (GeV/cγ µM2Entries / 20 GeV/c10-410-310-210-1110(a)0 100 200 300 400 500Dataγ) + τ τ →, µ µ →Z (Z+jettﬀWZ+ZZ+tﬁ2Entries / 20 GeV/c2Entries / 20 GeV/c)2ﬂ (GeV/cγ µ µM50 100 150 200 250 300 350 4002Events / 20 GeV/c10-310-210-1110Dataγﬃ) + τ  τ  →, µ! µ! →Z ("Z+jet"t#WZ+ZZ+t(b)ﬂ2Events / 20 GeV/cﬂ2Events / 20 GeV/cFIGURE 1. Background predictions and data observations for Mµγ (a) and Mµµγ (b). (GeV)γ$ T%EEvents / 10 GeV10-210-111015 25 35& 45 55' 65( 75 85) 95*Dataγ+) + τ, τ, →, µ µ →Z  (Z+jet-t.WZ + ZZ + t$%Events / 10 GeVFIGURE 2. Background predictions and data observations for photon ET .GM model, the total signal acceptance increases from 12% at Mµ / 100 GeV/c2 to 23% for Mµ   300 GeV/c2.BACKGROUND ESTIMATES AND DATA OBSERVATIONSThe µµγ signature can be produced by several standard model and detector sources: (1) Z 	 γ10 µµ 23 γ; (2)Z 	 γ 10  ττ 24 γ; (3) Z 0  µµ 24 jet, where a jet is misreconstructed as a photon; (4) t  ¯t  µνµνbb, where a fermionradiates a hig-ET photon; (5) W 0  eν 25 Z 0  µµ 2 and Z 0  ee 26 Z 0  µµ 2 , where an electron is misidentified as aphoton. The primary background Z 	 γ 10  µµ 27 γ is modeled using the ZGAMMA program [9]. The Z 0  µµ 25 jetis estimated using data. The total background prediction is 8 8 3 9 0 8 9 events (16 8 6 9 1 8 8 µγ combinations). In our ISRZ  γ control region, 81  Mµµ  101 GeV/c2 and EγT  50 GeV, we predict 7 8 4 :1 ; 20 ; 8 and observe 5 events.In the signal region, we observe 17 events with a background prediction of 8 8 3 9 0 8 9 events. The backgroundprediction and data are shown as a function of Mµγ in Figure 1(a). Several studies were done to understand theobserved excess. The Z  µµγ background, where the γ is produced via final-state radiation (FSR), is defined by81  Mµµγ  101 GeV/c2. In this region, we observe 11 events with a prediction of 5 8 5 9 0 8 5 events, as shown inFigure 1(b). As a check, we lower the ET cut to 15 GeV and observe 43 events, with a prediction of 38 8 5 9 4 8 0events, as shown in Figure 2. These studies indicate that the excess at low mass is consistent with an upward statisticalfluctuation, primarily in Z  µµγ FSR. There is no excess at high mass to indicate new physical processes.Mµ  LIMITS AND EXCLUSION REGIONSA Bayesian approach is used to obtain the upper limits on the experimental cross section at the 95% confidence level(C.L.). For Mµ < Λ (Mµ < Λ 	 f ) in the CI (GM) model, masses below 853 GeV/c2 (221 GeV/c2) are excluded,as shown in Figure 3. Because the GM exotic muon cross section depends on both Mµ  and f 	 Λ, we plot the two-dimensional f 	 Λ = Mµ  exclusion region in Figure 4(a). The excited muon CI model is valid for Mµ  	 Λ  1; we plotthe CI exclusion region in the Mµ  	 Λ = Mµ  plane in 4(b).)2ﬂ  (GeV/c*µM) (pb)γ> µ →* µ BR(⋅) µ* µ → p(p σ10-1100 300 500 700 9002212GeV/c2853 GeV/c? GM Theory*µ/f=M@Λ CI Theory*µ=MΛAGM 95% CL LimitBCI 95% CL LimitCFIGURE 3. Mass limits for Mµ DFE Λ (Mµ DGE Λ H f ) in the CI (GM) model.100I 150I 200J 250J 300K 350K 400L 450L10-410-310-210-1)M2 (GeV/c*µNMO)-1 (GeVΛf/10-410-310-210-1CDF Run II*µP = 2 M*µPΓOPAL, DELPHI100 200 300Q 400(a)R95% C.L.  Exclusion RegionMNO)-1 (GeVΛf/)S2 (GeV/c*µTM Λ ⁄U *µM00.2V0.4V0.6V0.8V1WST Λ ⁄U *µMVVVVW200X 400Y 600Z 800[95% C.L.\Exclusion Region(b)]FIGURE 4. Exclusions regions for the GM (a) and CI (b) models. Also shown in (a) are the regions excluded by the OPAL andDELPHI experiments for the GM model [10].CONCLUSIONWe have presented a search for excited and exotic muons in the µγ channel. No evidence of a µ  signal is found.Limits on the excited muon mass are established based on a contact interaction and a gauge-mediated model, the latterof which are the first limits at a hadron collider.ACKNOWLEDGMENTSWe are grateful to Alejandro Daleo for providing NNLO cross section calculations. We thank the Fermilab staff andthe technical staffs of the participating institutions for their vital contributions.REFERENCES1. U. Baur, M. Spira, and P. M. Zerwas, Phys. Rev. D 42, 815 (1990), and references therein.2. J.L. Hewett and T.G. Rizzo, Phys. Rep. 183, 193 (1989), and references therein.3. T. Sjo¨strand, Comput. Phys. Commun. 82, 74 (1994), version 6.127. We divide the µ  production cross section calculated byPYTHIA v6.127 by a factor of 2, to be consistent with PYTHIA v6.211, as described in the PYTHIA release notes.4. K. Hagiwara, S. Komamiya, and D. Zeppenfeld, Z. Phys. C 29, 115 (1985), and references therein; E. Boos et al., Phys. Rev. D66, 013011 (2002), and references therein.5. A. V. Semenov, hep-ph/0208011 (2002); A. V. Semenov, Comput. Phys. Commun. 115, 124 (1998).6. A. Pukhov et al., hep-ph/9908288 (1999); E. E. Boos et al., hep-ph/9503280 (1995).7. D. Acosta et al. (CDF Collaboration), Phys. Rev. Lett. 94, 101802 (2005).8. R. Brun and F. Carminati, CERN Program Library Long Writeup, W5013, 1993 (unpublished), version 3.15.9. U. Baur and E. L. Berger, Phys. Rev. D 47, 4889 (1993).10. G. Abbiendi et al. (OPAL Collaboration), Phys. Lett. B 544, 57 (2002); J. Abdallah et al. (DELPHI Collaboration),Eur. Phys. J. C 46 , 277 (2006).

Search for excited and exotic muons at CDF

Search for Excited and Exotic Muons at CDF

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Search for excited and exotic muons at CDF

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